Gate having voltage divider shunting series emitter-collector paths and individual base-bias level setting means equalizing transistor leakage currents



Nov. 10, 1964 J. BAUDE 3,156,831

GATE HAVING VOLTAGE DIVIDER. SHUNTING SERIES EMITTER-COLLECTOR PATHS ANDINDIVIDUAL BASE-BIAS LEVEL SETTING MEANS' EQUALIZING TRANSISTOR LEAKAGECURRENTS Original Filed Aug. 5, 1958 2 Sheet s-Sheet 1 I f Z/QEXZQ W Z6Z6 M -u- AH W 14 W 10 Nov. 10, 1964 v J. BAUDE. 3,156,831

GATE HAVING VOLTAGE DIVIDER SHUNTING SERIES EMITTER-COLLECTOR PATHS ANDINDIVIDUAL BASE-BIAS LEVEL SETTING MEANS EQUALIZING TRANSISTOR LEAKAGECURRENTS Original Filed Aug. 5, 1958 2 Sheets-Sheet 2 A 1/ I madam 6 M117 9 0 444, @zflmmole I United States Patent C) GATE HAVING VOLTAGEDIVIDER SHUNTING SElRlES EMlTTER-COLLECTOR PATHS AND IN- DIVIDUALBASE-BIAS LEVEL SETTING MEANS EQUALIZING TRANKSTOR LEAKAGE CUR- RENTSJohn Baude, Milwaukee, Wis, assignor to Allis-Chalmers ManufacturingCompany, Milwaukee, Wis.

Original application Aug. 5, 1958, Ser. No. 753,272, now Patent No.3,656,043, dated Sept. 25, 1962. Divided and this application Mar. 23,1962, Ser. No. 181,997

2 Claims. ((31. 367-885) This invention relates to electric switches andmore particularly to electric switches wherein a semiconductor elementis used as a controlling device.

The development of commercially useful semiconductor devices such astransistors for switching purposes has many advantages over the priorart manual or magnetically triggered devices for opening or closing acircuit in response to variations in circuit conditions. Transistors aresmall in size, durable and consist of material having a long usefullife. They respond rapidly to a triggering force and open and close thecircuit without the use of moving contacts.

The use of power transistors for direct current control systemsexceeding 80 to 100 volts is seriously handicapped by the fact thatpresent day power transistors with a voltage rating exceeding 80 to 100volts are very expensive. These voltage ratings are maximum ratingswhich cannot be exceeded for even short time intervals without damage tothe semiconductor material of the transistors. The current carryingcapacity of the present day transistors are limited by the currentcarrying ability of their leads and the heat losses within theirsemiconductor materials. The current carrying characteristics of thetransistors, however, render them useful for switching purposes whereinshort time overloading may occur but definitely preclude their use ifthe transistors are subjected to voltage surges at a value above theirvoltage ratings. Any control system or electric switch utilizing aplurality of transistors in a circuit exceeding the maximum voltagerating of each individual transistor must provide close control of thevoltages applied to the transistor terminals.

In accordance with the invention claimed a new and improved electricswitch is provided comprising first and second transistors each having abase, an emitter and a collector. Means are provided for connecting theemitter and collector of the transistors in a series circuit across asource of electric potential. Impedance means are connected across eachof the transistors from their emitter to their collector electrodes forcontrolling the voltages applied to the transistors. Means are connectedto the base of each of the transistors for controlling conductionthrough the emitter collector circuit.

It is, therefore, one object of this invention to provide a new andimproved transistor switch.

Another object of this inveniton is to provide a new and improvedtransistor switch for use in either an alternating or direct currentcircuit.

A further object of this invention is to provide a new and improvedstatic control switch utilizing transistors having voltage ratings belowthe voltage rating of the controlled electric circuit.

A still further object of this invention is to provide a new andimproved static control system employing transistors which divides thevoltage drop proportionally across each of the transistors when theirbases are positive with respect to their emitters.

Objects and advantages other than those set forth will be apparent fromthe following description when read in connection with the accompanyingdrawings, in which:

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FIG. 1 is a schematic view of a transistor switch embodying theinvention;

FIG. 2 is a modification of the transistor switch shown in FIG. 1wherein the dotted showing of the resistors are a further modificationof the invention;

FIG. 3 is a further modification of the transistor switch shown in FIG.1 illustrating a compound connection of the transistors embodying theinvention;

FIG. 4 is a still further modification of the transistor switch shown inFIG. 1 embodying the invention wherein the transistors are connected inseries across an alternating current source; and

FIG. 5 is a modification of the tnansistor switch shown in FIG. 4wherein a transformer is used for providing control bias for the baseelectrode circuit.

Referring more paritcularly to the drawings wherein like parts aredesignated by like characters of reference throughout the figures, FIG.1 illustrates a pair of junction transistors 10 and 10' connected inseries with a load 11 across a source of direct current potential 12,12. Each transistor comprises a body of semiconducting material whichmay be composed of silicon, germanium or any other suitable materialcontaining minute quantities of atomic impurities. In addition to thebody, the transistors comprise emitters 13 and 13', collectors 14 and14' and bases 15 and 15. The circuit shown in FIG. 1 covers the use ofPNP type transistors but can be modified for NPN type transistors if thetransistors are properly connected for reverse polarities. As noted fromFIG. 1 the collector 14 of transistor 10 is connected to the emitter 13'of transistor 16 and the collector 14' of transistor 19 is connected tothe load 11. Impedance means 16, 16' are connected one across each ofthe transistors 10, 10' from the emitter to the collector of each ofthese transistors. These impedance means comprise resistors 17, 17'connected in parallel with capacitors 18, 18', respectively. Resistors17, 17 function to keep the voltage drop across each tnansistor fromemitter to collector equal when the base of each transistor is positivewith respect to the emitter. The resistance of resistors 17, 17' isselected such that the total leakage current through the circuit fromterminal 12 to terminal 12' at cutolf is only slightly increased overand above the transistor leakage current. The capacitors 13, 18' areused to prevent running away of one transistor with reference to theother one under switching conditions. That is, they act as buffers toslow the response of the transistors during switching from conducting tononconducting condition or vice versa.

The emitters 13 and 13 of transistors 10,. 10', respectively areconnected to bases 15, 15' through a suitable potential source shownhere as transformers 19, 19'. The outputs of the secondary windings 2t29 of transformers 19, 19 are rectified by a pair of diodes 21, 21'. Theprimary windings 22, 22' of transformers 19, 19 are connected across asuitable source of alternating current 23. The center tap of each of thesecondary windings 26, 2% are connected through resistors 24, 24' tobases 15, 15' of transistors 13, 10. A pair of resistors 25, 25 areconnected one between the emitter and base of each transistor to helpkeep the transistors at cutoff until a control current is applied to thetransistors base. Capacitors 26, 26 connected in parallel with resistors25, 25', respectively, are used to eliminate excessive surges that mayenter through transformers 1d, 1d. When transformers l9, 19' areenergized from the alternating current source tap of the secondarywindings 20, 20' of transformers 19, 19'.

In accordance with the invention claimed an impedance means is connectedacross the emitter and collector of each transistor. The resistors actas voltage dividers and under cutolf conditions will divide the voltageacross each transistor properly. Under conducting condition of thetnansistors the transistors will have substantially the same voltagethereacross and will pass the same amount of current if they are closelymatched.

Under switching conditions from on to off there is the possibility thatone transistor may conduct current quicker than the other one. Thisoccurs if the base con trols or transistors are not perfectly matched.Therefore, to eliminate or substantially reduce the differences inconducting time or current amplitude of the serially connectedtransistors the capacitors 18, 18' and 26, 26 are utilized to delaytheir response. Capacitors 18, 18 react to sudden changes in voltagelike a short circuit. Since all of the circuit elements are essentiallyidentical in size and performance, capacitors 18, 18' will dischmgesubstantially simultaneously into their respective transistors. In viewof the fact that transistors have a tremendous overcu-rren't rating noharm is done to the transistors by a discharge of these capacitors. Whenthe transistors are triggered from conducting to nonconducting conditionas in the situation when control power from source 23 is removed, theserially connected transistors are stabilized by capacitors 26, 26 whichdischarge their energy substantially simultaneously into resistors 25,25', respectively. Capacitors 18, 18 also react as stabilizers duringthe transition period of the transistors from conducting tononconduc-ting or cutoff condition.

Although it is possible to control the power transistors by applicationof direct current potential individually to each base, from :acommercial View point it may be more desirable to supply the basepotential from an alternating current source through a transformer thesecondary winding of which is rectified and then supplied to the basesof each of the transistors, as shown in FIG. 1.

When the transistors 10, lit in the triggering circuit existing betweenterminals 12. and 12 of the direct current source are on", the bases 15,15 thereof are close to zero potential with respect to the emitters 13,13. Upon a signal impulse being applied to the transformers '19, 19' thebases of transistors 10, 1d are rendered negative and the transistorsconducting. Because of the disclosed invention transistors may be usedas switches to control potential source having a voltage ratingexceeding the voltage rating of any one of the transistors used.

FIG. 2 illustrates a modification of the circuit shown in FIG. 1 whereineach of the transistors it are triggered directly by a pair of directcurrent sources 23, 28 connected between the emitter and baseelectrodes. Further, the series connected emitter collector circuitcomprises a pair of diodes 29, 29' one connected in series with each ofthe emitters between the emitters and their terminal connections withdirect current sources 28, 28'. As shown in full lines, resistors 17, 17are each connected as in FIG. 1 from the emitter to the collectorelectrodes.

In order to render the bases of transistors 19, to more positive withreference to their emitters and thereby control the leakage currentflowing from the collectors through the bases to the emitters and toprevent thermal runaway if the temperature of the transistors shouldrise, diodes 29, 29' are utilized in series with the emitters as shownin FIG. 2. If desired, resistors 17, 17 may be arranged across thediode, emitter and collector circuit as shown in dotted lines in FIG. 2.In the full line connection of the resistors 17, 17 the diodes 29, 29'are each in series with the flow of current through the resistors 17,17' causing a voltage drop across the diodes and thereby rendering thebases of the transistors more positive than the emitters. This cur-rentis independent of the leakage current through the transistor and imposesa bias upon the transistor which is not responsive to temperature. Inthe dotted illustration of the resistors 17, 17' the leakage current isreduced and retarded so that the voltage drop does not appear across thediodes until current flows through the diodes. Therefore, the diodes 29,2% are not as elfective in rendering the bases more positive than when:the resistors '17, 17 are connected as shown in the dotted: positions inFIG. 2.

F IG. 3 is a modification of the transistor switch structure shown inFIGS. 1 and 2 and comprises a compound con-- nection of a plurality oftransistors embodying the invention. Two transistors may be connected inthe compound connection to obtain a single three terminal unit. Thisform of connection may be used for obtaining a transistor arrangementhaving an emitter to collector current amplification factor ofapproximately 1. As the current is creased to the first of two compoundconnected transistors more current is diverted to the second transistor,the output current of this compound arrangement being the sum of thecollector currents of the two transistors. The load sharing propertiesof this arrangement is utilized in combination with the invention asshown in FIG. 3.

In FIG. 3 transistors 30, 30' have their emitters 31, 51' connected tothe bases 15, 15 of transistors 1d, 14) and transformers 19, 19',respectively. The collectors 32, 32 of transistors 39, 30 are connectedto the collectors 14, 1 5 of transistors 10, 10 in the usual manner forthis type of compound arrangement. Bases 33, 33' are connected throughresistors 24, 2d and diodes 2d, 21', respectively, to one side of thesecondary windings 2t 20 of transformers 19, 19. Tie other side of thesecondary windings 2t 2% are connected to the emitters 31, 3-1 oftransistors 36?, 359', respectively. Resistors 34, 34' are pr vided forconnecting the bases of transistors 10, it) to their respective emitters13, 13' to provide a cutoff bias as previ-' ously explained withreference to FIG. 1. The circuit shown in FIG. 3 requires much lesscontrol power to carry the same load current of the transistors shown inFlGS. 1 and 2 and, therefore, proves to be more stable than the circuitsshown in FIGS. 1 and 2. With the circuits shown in M68. 1 and 2 a changein the load circuit which would change the base drive requirements couldresult in failure of the transistor due to the increased voltage dropacross the emitter and collector and the resulting greater heatdissipation within the transistor. if for some reason the transistors10, to are over-loaded part of the load current will be diverted totransistors 30, 33. Thus, in this compound arrangement a high degree ofamplification of the control current occurs and the circuit will remainstable because the load current will be divided between the transistorsdd, 10' depending on the voltage drop across the emitter and collectorof transistors it), 10'.

FIG. 4 illustrates a further modification of the invention shown inFIGS. l3 wherein an alternating current tandem operation of twotransistors is used for switching purposes. As shown in MG. 4 thecollectors 14, 14 of transistors it til are connected together withtheir emitters l3, 13', in series with load 11 across the alternatingcurrent source 2, 2'. Impedance means 16, 16' comprising resistors 17,17' connected in parallel with capacitors 13, 18' are connected onebetween the emitter and collector electrodes of each transistor 14),1t). Bases 15, 15' are connected to the positive side of the directcurrent source 28. A transistor 37 is connected with its emitter 38 tothe base 15 of transistor 10 and its collector 3% to the negativeterminal of a direct current source 443. The positive terminal of source46 is connected to the interconnected collectors 14, 14 of transistors1d, lit). Base 41 of transistor 37 is connected to the negative terminalof. source 28. By using the source so in series with the collectoremitter circuit of transistor 37 in the base control circuit oftransistors 10, 10' amplification of the signal from source 28 isobtained.

FIG. 5 illustrates a modification of the transistor con-- trol circuitshown in FIG. 4 wherein a transformer 42 arranged across the alternatingcurrent source 2, 2' is provided for supplying the direct currentpotential to a NPN transistor 43. Transistor 43 has its emitter 44connected in series with one end of the secondary winding 52 oftransformer 42 through a diode 45. The center top of winding 52 isconnected to the interconnected collectors 14, 14 of transistors 10,iii. The other end of winding 52 is connected through a diode 45 to theseries connection of diode i5 and emitter 44. The collector d6 oftransistor 43 is connected to the common base connection of transistors1t 1t? and the positive terminal of the source 23. The base 47 oftransistor 43 is connected to the collector 43 of PNP transistor 49. Thebase 50 of transistor is connected to the negative terminal of thesignal source 23. The emitter 51 of transistor 49 is connected to thepositive terminal of source 26.

The circuit illustrated in FIG. 5 is a practical appli cation of thebasic transistor control principle disclosed for controlling analternating current circuit. By using an NPN type transistor in thecontrol circuit provides easy amplification of the control signal.

If desired, the emitter and collector connection of transistors 10, 145'shown in FIGS. 4 and 5 may be reversed thereby requiring less controlpower for controlling the switching function formed by thesetransistors. However, the conductivity of this circuit is not aspronounced when the control power or signal is applied between the baseand emitter electrodes as compared to applying it between the base andcollector electrodes of transistors 10, 10.

Although but a few embodiments of the present invention have beenillustrated and described it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

This application is a division of application Serial No. 753,272, nowPatent No. 3,056,043, filed August 5, 1958 by John Eaude and entitledGate Having Voltage Divider Shunting Series Emitter-Collector Paths andIndividual Base-Bias Level Setting Means Equalizing Transistor LeakageCurrents.

Having now particularly described and ascertained the nature of my saidinvention and the manner in which it is to be performed, I declare thatwhat I claim is:

1. In an electric switch, first and second transistors each having abase, an emitter and a collector, means for connecting said emitter andcollector of each of said transistors in series circuit across a sourceof electrical potential, impedance means connected across each of saidtransistors from said emitter to said collector for controliing thevoltage applied to said transistors, and means connected to said base ofeach of said transistors for controlling conduction through said emittercollector circuit, said means comprising a third and fourth transistoreach having a base, emitter and collector, said emitter of each of saidthird and fourth transistors being connected to a different base of saidfirst and second transistors, said emitter and base of each of saidthird and fourth transistors being connected together across a source ofelectrical potential, said collector of each of said third and fourthtransistors being connected to the collector of one of said first andsecond transistors.

2. in an electric switch, first and second transistors each having abase, an emitter and a collector, means for connecting said emitter andcollector of each of said transistors in series circuit across a sourceof electrical potential, impedance means comprising a resistor and acapacitor connected in parallel and across each of said first and secondtransistors from said emitter to said collector for controlling thevoltage applied to said first and second transistors, and meansconnected to said base of each of said first and second transistors forcontrolling conduction through and voltage applied to said emittercollector circuit, said means comprising a third and fourth transistoreach having a base, emitter and collector, said emitter of each of saidthird and fourth transistors being connected to a different base of saidfirst and second transistors, said emitter and base of each of saidthird and fourth transistors being; connected together across a sourceof electrical potential, said collector of each of said third and fourthtransistors being connected to the collector or" a different one of saidfirst and second transistors.

References Cited in the file of this patent UNITED STATES PATENTS2,533,001 Eberhard Dec. 5, 1950 2,957,993 Sichling Oct. 25, 1960 FOREIGNPATENTS 536,516 Great Britain May 16, 1941

1. IN AN ELECTRIC SWITCH, FIRST AND SECOND TRANSISTORS EACH HAVING ABASE, AN EMITTER AND A COLLECTOR, MEANS FOR CONNECTING SAID EMITTER ANDCOLLECTOR OF EACH OF SAID TRANSISTORS IN SERIES CIRCUIT ACROSS A SOURCEOF ELECTRICAL POTENTIAL, IMPEDANCE MEANS CONNECTED ACROSS EACH OF SAIDTRANSISTORS FROM SAID EMITTER TO SAID COLLECTOR FOR CONTROLLING THEVOLTAGE APPLIED TO SAID TRANSISTORS, AND MEANS CONNECTED TO SAID BASE OFEACH OF SAID TRANSISTORS FOR CONTROLLING CONDUCTION THROUGH SAID EMITTERCOLLECTOR CIRCUIT, SAID MEANS COMPRISING A THIRD AND FOURTH TRANSISTOREACH HAVING A BASE, EMITTER AND COLLECTOR, SAID EMITTER OF EACH OF SAIDTHIRD AND FOURTH TRANSISTORS BEING CONNECTED TO A DIFFERENT BASE OF SAIDFIRST AND SECOND TRANSISTORS, SAID EMITTER AND BASE OF EACH OF SAIDTHIRD AND FOURTH TRANSISTORS BEING CONNECTED TOGETHER ACROSS A SOURCE OFELECTRICAL POTENTIAL, SAID COLLECTOR OF EACH OF SAID THIRD AND FOURTHTRANSISTORS BEING CONNECTED TO THE COLLECTOR OF ONE OF SAID FIRST ANDSECOND TRANSISTORS.